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Tentative testdriver methods #226

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Tentative testdriver methods
jcsteh Jun 3, 2025
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Add additional risks per review.
jcsteh Jul 3, 2025
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Cross-reference documentation on how to mark a test tentative.
jcsteh Jul 23, 2025
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Clarify that this is about WebDriver endpoints not defined in any spe…
jcsteh Jul 23, 2025
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32 changes: 32 additions & 0 deletions rfcs/tentative_testdriver_methods.md
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# RFC 226: Tentative testdriver methods

## Summary
Allow tentative methods to be added to testdriver.js which use WebDriver endpoints not yet defined in a specification.
This should only be allowed where there is consensus between multiple browser vendors and where there is clear progress towards a specification.

## Background
As part of the [Interop 2025 Accessibility Investigation Area](https://github.com/web-platform-tests/interop-accessibility/issues/148), the intention is to extend browsers and WPT to allow additional accessibility properties to be tested by web platform tests.
However, there are open questions regarding the shape of the API for exposing these properties, what properties should be exposed, how the tests should be written, etc.
While the end goal is to extend the WebDriver specification with the required new endpoints, these open questions need to be answered before this is feasible.
To answer these questions, it would be helpful if browser vendors could collaborate on a tentative but working implementation.
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For things like the Accessibility Investigation, is there any intention for there to be any, even high-level, explainer of what the proposed endpoints are? I'm mostly concerned about the risk of ending up somewhere where it is hard for anyone else to start passing the tests written using the tentative endpoint.

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There is WICG/aom#203. However, that's an evolving issue, not a document. I could create an explainer document if that's what we need, though.


## Details
1. There must be publicly documented (e.g. as part of an Interop Investigation Area) consensus between at least two browser vendors and intent to work towards a specification.
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I agree that the tests for these tentative methods should be marked with tentative. But I think we should treat these tentative methods similarly to how we treat tentative tests for the sake of consistency. Given tentative tests do not require consensus currently, I would remove this requirement from tentative methods.

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I'd argue that tentative testdriver API pose a greater risk, because removing tests that rely on undocumented API is painful and hard — see #172.

2. Any tentative methods added to testdriver.js should be prefixed with `tentative_`.
3. Any utility methods which call tentative testdriver methods should also be prefixed with `tentative_`.
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I wonder if we want there to be a tentative object, or whether we just want to prefix everything like this?

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I wonder if having a tentative object would make it trickier to "graduate" things from tentative once the spec is final (and my thinking is that we want to incentivise finalising tentative things as soon as is feasible). With prefixing, you can basically just search/replace once the spec is final. With an object, things will need to be moved around. That's obviously not a show stopper if an object is preferred, just something I thought worth considering.

4. Any tests directly or indirectly calling tentative testdriver methods must be [marked tentative](https://web-platform-tests.org/writing-tests/file-names.html#:~:text=.tentative,-:%20).

## Alternatives considered
1. Avoid any changes to the WPT repository until the specification is finalised.
This makes it very difficult for browser vendors to collaborate and to learn about problems that are much easier to discover and understand "in practice".
In contrast, if the specification were developed without a working implementation, there is a much higher risk of fundamental design problems in the specification which are much harder to fix later.
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Perhaps the initial cross-engine prototype of the property bag implementation should include only the properties already exposed by other WebDriver methods, like computedrole and computedlabel.

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This would make it easier to standardise on a first pass, for sure. However, that first pass would have no practical benefit, since we can already get those properties using the existing methods. The problem I see with this is that we're less likely to spot problems because the implementation is thus entirely "theoretical": we won't spot the kinds of problems we're likely to see in testing new properties, which is the entire point of this work. In contrast, the current prototype supports a few simple ARIA properties and parent/children, which allows us to test the kinds of things we really want to test with this work and thus spot problems we might hit as we move forward.

2. Develop the implementation as vendor specific tests with methods in testdriver-vendor.js.
While this does allow a single vendor to iterate on a working implementation, it is still very difficult for multiple browser vendors to collaborate on this.
At the very least, other vendors may wish to contribute tests which exercise the implementation to ensure it fits their needs before agreeing to a final specification.

## Risks
1. Methods could be added which never end up being specified, resulting in cruft and non-standardised functionality.
This is not a significant risk to the web at large because these methods only impact tests and the tests must be marked tentative, preventing them from being considered for Interop scoring, for example.
2. The API could change significantly before it becomes finalized, with many tests depending on the tentative API.
This could mean that migrating the tests to the final API requires significant effort.
3. Because of the lack of specification, other vendors interested in ensuring interoperability with the feature being tested might have to reverse engineer how the tentative API is intended to function.